As Romney formally announced Ryan in Norfolk, Va., deputy Obama campaign manager Jim Messina blasted out an email statement casting the Republican ticket as a potential disaster for the middle class and seniors.

"In naming Congressman Paul Ryan, Mitt Romney has chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy," Messina wrote.

"As a member of Congress, Ryan rubber-stamped the reckless Bush economic policies that exploded our deficit and crashed our economy," he said. "Now the Romney-Ryan ticket would take us back by repeating the same, catastrophic mistakes."

A Democrat with ties to the Obama campaign also said privately, hinting at the message to play out in the days ahead, that Romney has "capitulated to the extreme in his party."

Over the past few months, Democratic researchers at both the campaign and independent groups have been amassing opposition files on potential VP candidates, including Ryan, for a rapid and robust response.

One of those groups, the liberal PAC American Bridge, today launched a new web site - MeetPaulRyan.com - featuring hundreds of pages of critical background on Ryan and hours of tracking footage.

Obama campaign officials would not formally comment on the Ryan pick ahead of Romney's announcement. But the Democratic National Committee signaled it would bracket the moment with a planned shadow bus tour through Virginia and North Carolina.

Democratic strategists say Ryan will be a liability for Romney across demographic groups, with focus groups and internal party polling pointing to a strong negative public opinion of Ryan's budget, particularly his plan to revamp Medicare. "

The Obama campaign's liberal allies jumped on the selection of Ryan as "extreme" and a "major unforced error."

"It gives President Obama and Democrats a chance to draw a clear contrast in 2012 by promising not to cut one penny from Medicare or Social Security benefits. If Democrats win in a landslide, this was the game changer," said Adam Green, co-founder of the Progressive Change Campaign Committee, a grassroots group.

Mary Kay Henry, president of the SEIU, one of the nation's largest unions, added that "Romney has shown the American people that he believes Rep. Ryan's extremist, irresponsible and anti-worker agenda is what's right for our country."

"Rep. Ryan has made a name for himself by fighting in the halls of Congress for tax giveaways for the wealthy and big corporations while proposing to gut vital services like Medicare and education, and eliminating any sense of retirement security for working families," she said.